Virus

[from the analogy with biological viruses]
A cracker program that searches out other programs and 'infects' them by embedding a copy of itself inside, so that they become a Trojan. When these programs are executed, the embedded virus is executed as well, thus propagating the 'infection'. This normally happens invisibly to the user. Unlike a worm, a virus cannot infect other computers without assistance from the cracker.

Spyware

[a type of malware] Undetected software that can be installed on computers which collects small pieces of information on users without their knowledge. Typically, spyware is secretly installed on the user's personal computer with similar malicious software such as keyloggers by the owner of a shared, corporate, or public computer on purpose in order to secretly monitor other users.

Trojan Horse

[coined by MIT-hacker-turned-NSA-spook Dan Edwards]
A malicious, security breaking program that is disguised as something benign, such as a directory lister, archiver, game, picture, or even a program to find and destroy viruses.

Malware

[short for malicious software] Programming (code, scripts, active content, and other software) designed to disrupt or deny operation, gather information leading to loss of privacy or exploitation, gain unauthorized access to system resources and other abusive behavior.

Worm

[from 'tapeworm']
A program that propagates itself, reproducing as it goes until it fills all of the storage space on the selected drive or network.